Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.
When the rear window defogger switch is activated, the radio/HVAC control panel sends a serial data message to the HVAC control module requesting the rear defogger be activated. The HVAC control module responds by energizing the rear defog relay by applying voltage to the rear defog relay control circuit. Ground for the relay coil is provided at ground G104. When the rear defog relay coil is energized, the switch contacts of the rear defog relay close allowing battery voltage to flow from the REAR DEFOG fuse through the closed switch contacts and out the relay switched output to the rear window defogger grid. Ground for the rear window defogger grid is provided at grounds G401 and G402.